Heretofore, various attempts have been made to design safety latch mechanisms for pressure vessels, which serve to prevent unlocking of a vessel closure until pressure within the vessel has been reduced to a safe level or to prevent pressurization of a vessel until a closure therefor is in a desired locked condition, and as by way of illustration, reference can be made to U.S. Pat. Nos. 1,208,041; 1,333,262; 1,470,371; 2,596,353; 2,706,999 and 3,830,400 and British Pat. No. 412,700.